202 FLAVONOIDS AND RELATED COMPOUNDS 



ments of Harborne (96) support this idea. If one assumes a conversion of leucoanthocy- 

 anidins to anthocyanins, it is indicative that the former are never glycosylated while the 

 latter always are. By analogy with the methoxy groups of other natural products (pectin, 

 alkaloids) it is probable that methionine serves as a methyl donor for the flavonoids and 

 related compounds. However, no tracer experiments have been carried out to support 

 this. 



Biosynthesis of the phlobatannins has not been investigated by the tracer technique. 

 The consensus is that they are formed by polymerization of catechin or leucoanthocy- 

 anidin monomers (97). Since the exact structure of the phlobatannins is unknown, differ- 

 ent mechanisms for the condensation have been suggested. Freudenberg's condensation 

 (see p. 178) is non-oxidative. Others have proposed an oxidative condensation either non- 

 enzymatic or catalyzed by a peroxidase or phenol oxidase. These oxidative mechanisms 

 have been studied by Hathway (98, 99, 100, 101) who favors a quinoid structure for the 

 polymer: 



Hathway believes that the formation of phlobatannins is normally catalyzed by phenol oxi- 

 dase of the cambium but may also be nonenzymatic. Herz (102) has found that peroxidase 

 of blood can catalyze a similar conversion. Roux and Evelyn (103) suggest that the cambi- 

 um furnishes precursors which are converted to specific condensed tannins in other parts 

 of the plant - even in the central (dead? ) heartwood. 



GENERAL REFERENCES 



Bate-Smith, E. C. "Flavonoid Compounds in Foods" Advances in Food Research 5 261 

 (1954). 



Bentley, K. W. , The Natural Pigments , Interscience Publishers, N. Y. , 1960. 



Blank, F. "The Anthocyanin Pigments of Plants", Bot. Rev. 13 241 (1947). 



Blank, F. "Anthocyanins, Flavones, Xanthones", in Ruhland 10 300. 



Freudenberg, K. , Tannin, Cellulose , Lignin , Springer, Berlin, 1933. 



Freudenberg, K. and Weinges, K. "Catechine, andere Hydroxyflavane und Hydroxy- 

 flavene", Fortschr. Chem. Org. Naturstoffe 16 1 (1958). 



Freudenberg, K. and Weinges, K. , "Systematik und Nomenklatur der Flavonoide" Tetra- 

 hedrons 376 (1960). 



Geissman, T. A., "Anthocyanins, Chalcones, Aurones, Flavones and Related Water- 

 Soluble Plant Pigments", Paech and Tracey, 3 450. 



Geissman, T. A. , ed. , The Chemistry of Flavonoid Compounds , Macmillan, New York, 

 1961. 



